1. Field of the Invention
The present invention relates generally to the field of electron (i.e., .beta. radiation) beams. More particularly, the present invention relates to an addressable field emitter array that produces a spatially modulated electron beam matrix. Specifically, a preferred implementation of the present invention relates to a digitally addressable field emitter array in combination with an electrostatic acceleration grid and a magnetic lens assembly that includes a telecentric magnetic lens system and a correction lens system which together de-magnify the spatially modulated electron beam matrix onto a target wafer so as to directly write an electron resist with minimal distortion. The present invention thus relates to an addressable field emitter array of the type that can be termed lithographic.
2. Discussion of the Related Art
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.
Historically, optical lithography for semiconductor wafer production has been limited in resolution by the wavelength of the light source being used. At any particular wavelength of light, diffraction limits the focusability of an optical beam to a circle of a diameter approximately equal to the wavelength of the light source. Consequently, the fabrication industry has evolved several generations of lithographic light sources, each using a shorter wavelength than its predecessors. As microelectronic features have continued to shrink, the resolution of optical lithography is now a barrier to further increasing the density of chip features because suitable light sources of still shorter wavelength are no longer readily available. What is needed therefore is a way to increase the resolution of lithography.
A previously recognized solution has been to use an electron beam to expose an electron resist that has been coated on the surface of a semiconductor wafer, thereby enabling a pattern to be etched into the surface of the wafer through apertures formed in the subsequently developed resist. Electron beam technology is already a strong candidate for lithographically producing semiconductor devices with line widths on the order of approximately 0.1 micron. Electron beam imaging resolutions on the order of approximately 80 .ANG. have been previously reported..sup.(2)
Prior art electron beam devices of the type hereunder consideration, sometimes called electron guns, are well-known to those skilled in the art. A conventional electron beam is typically generated in a vacuum by electrical resistance heating of a suitable material to generate a stream of electrons. This stream of electrons is then electrostatically and/or magnetically focused. Two specific prior art electron beam devices are the cathode ray tube (CRT) and the scanning electron microscope (SEM), both of which generate and focus a beam of electrons in a vacuum.
For example, referring to FIG. 1, a conventional CRT electron gun with a bipotential lens structure is shown where an electron beam 110 is incident a screen 120 at a potential V.sub.s..sup.(1) This conventional electron gun includes a cathode 130 and a first apertured grid 140 which is maintained negative with respect to the cathode and controls the flow of electrons from the cathode. A second apertured grid 150 is located downstream of a cross over point and set at a positive voltage with respect to the cathode 130 so as to attract the electrons and shape the beam 110. A focus electrode 160 focuses the beam 110. The resolution can be improved by using an electromagnetic focus instead of the focus electrode 160.
Referring now to FIG. 2, the principle of electromagnetic deflection of an electron beam is illustrated where a flux of electrons 210 is incident a screen 220 at a deflection amplitude. An electromagnetic deflection coil 230 is composed of two perpendicular windings generating electromagnetic field perpendicular to the trajectory of the electron beam in the vertical and horizontal planes. A field of length l is applied perpendicularly to the flux of electrons 210 which have previously been accelerated to a velocity V.sub.B. The flux of electrons 210, assuming the field intensity is uniform and of length l, is deflected onto a circular path of radius r. The corresponding angle of deflection is .theta. such that ##EQU1## where Ni is the number of ampere turns generating the magnetic field, D is the diameter of the cylindrical winding generating the field, l is the length of the field and V.sub.B is the accelerating voltage expressed in volts.
Although the electrons in the beams generated by the prior art electron beam devices have a wavelength, and a corresponding resolution limit that is imposed by diffraction, as a practical matter, the electron wavelength is so short that the diffraction of the electrons does not limit the resolution of the electron beam. This is because the prior art electron beam systems all include electrostatic lenses and/or magnetic lenses that generate stray fields. These stray fields are electromagnetic aberrations that result in distortions of the shape of the electron beam of a size that is at least on the order of nanometers. These distortions are larger than the wavelength of the electrons. Thus, the electromagnetic aberrations of the prior art devices limit the maximum resolution of the beam before the theoretical diffraction limit becomes an issue. Therefore, what is also needed is a way to reduce the effect of electromagnetic aberrations of the resolution of electron beam lithography.
Moreover, in the past, electron resist imaging has been relatively inefficient because the prior art electron beams have merely illuminated a single area and have had to be methodically scanned to write any sort of pattern. For example, referring to FIG. 3, a conventional fixed electron beam 310 is shown being truncated by a beam-shaping mask 320. Mask 320 includes a square shaped aperture. The mask 320 functions as a .beta. radiation shield and gives the electron beam a square cross-section which is more useful for exposure of an electron resist. The square beam is then shaped by a lens 330 and subsequently deflected by a scanning deflector 340 before striking a target wafer 350 on which an electron resist is coated.
Still referring to FIG. 3, to scan the illustrated T-shaped pattern, either the electron beam must be scanned by the operation of the scanning deflector 340, or the target wafer 350 must be moved. In any event, this requires a large amount of time to image the design of even a simple integrated circuit (IC). As the complexity of integrated circuits (ICs) increase, more demanding semiconductor circuit design rules require correspondingly smaller electron beam spot sizes. And the smaller the spot size, the more time is required to complete the scanning of a given pattern. Thus, electron beam lithography has heretofore been slow, and therefore expensive. Therefore, what is also needed, is an electron resist writing method having higher efficiency.
Still referring to FIG. 3, another problem with electron beam lithography has been that using a truncated electron beam involves the use of the mask 320 which absorbs significant energy from the screened electron flux. The thermal management of the mask 320 is problematic. This has prevented the use of any but the most simple shielding masks because masks with almost any degree of detail rapidly deform and melt due to their function as .beta. radiation shields. This problem is exacerbated by the fact that as higher energy electrons having shorter wavelengths are used, the thermal energy that needs to be dissipated also increases. Therefore, what is also needed is a way to tailor the cross-section of an electron beam without using a mask.
The below-referenced U.S. patents disclose embodiments that were satisfactory for the purposes for which they were intended. The disclosures of both the below-referenced prior U.S. patents, in their entireties are hereby expressly incorporated by reference into the present application for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art.
U.S. Pat. No. 3,665,241 discloses a field ionizer and field emission cathode structures and methods of production. U.S. Pat. No. 5,363,021 discloses a massively parallel array cathode.